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The Science of Tempering Chocolate

October 13, 2017

Which dessert can take shape as a chip in your favorite cookie or as a bar, and can be enjoyed in both a liquid and solid form? That’s right- it’s chocolate! Most of the time that you take a bite of chocolate, however, you probably don’t think about how a peanut-sized cocoa bean was crafted into the smooth, shiny, and sweet dessert of your dreams.

The complex transformation process includes a variety of chemical reactions among the approximately 380 chemicals that comprise it (Kuwana, 2010). In this article, you are going to learn about one such process that highlights chocolate’s unique chemical properties and how you can replicate this fundamental technique on your own.

The word temper meaning to heat to make hard applies to chocolate by making it solid at room temperature and perfectly melted you begin to chew it. As a chocolate lover, you should be very appreciative of tempering because the cocoa butter can solidify into 6 different kinds of fat crystals, however only two of these structures produce a stable/hard chocolate that is palatable (McGee, 2004). To ensure that the ideal texture is achieved, tempering is needed.

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Tempering generally follows a three-step process of heating the chocolate to dissolve the fat crystals, lowering the temperature slightly to form a new crystal structure, and reheating to dissolve all unstable crystals. This way you are left with only the desirable and stable crystals, which can then control the development of the crystal network upon the solidification of the chocolate (McGee, 2004). The stable cocoa butter crystals have a higher melting point meaning they form at a higher temperature than that of unstable ones. This follows the same principles of the method of slow-cooling crystallization where a rapid drop in temperature doesn’t provide the stable crystals with enough time to assemble before the unstable ones are already forming. Basically, the key to tempering is keeping the chocolate temperature below the melting point of stable cocoa butter crystals, but above that of unstable ones (this varies depending on the type of chocolate and the ratio of cocoa and milk fats).

Try it Yourself:

 What you need:

  • candy thermometer
  • saucepan and glass bowl (or a double boiler)
  • milk chocolate

The Recipe (extracted from On Food and Cooking):

  • To temper the chocolate from scratch, heat it to 40-45°C to melt all crystals, and cool it down to around 27-28°C. Then either stir the chocolate as it cools further, until it thickens noticeably (an indication of crystal formation), or pour a portion onto a cool surface and scrape and mix it until it thickens, and return it to the bowl. Then carefully raise the temperature of the chocolate to the tempering range, 30-31°C, and stir to melt any unstable crystals that might have formed during the stirring or scraping.

References:

Kuwana, Ellen. “Discovering the Sweet Mysteries of Chocolate.” Neuroscience For Kids,     Eric Chudler, 01 October 2010, faculty.washington.edu/chudler/choco.html.

McGee, Harold. On Food and Cooking: the Science and Lore of the Kitchen. Scribner, 2004.

Tomazincic, Sara. “Dessert Hungry.” Dessert Hungry, 06 October 2014, desserthungry.blogspot.com/2014/10/tempering-chocolate-caramel-and-almond.html.

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Caroline Culpepper
About Author

Caroline is an undergraduate Biochemistry major at Georgia Tech, and is a member of the Georgia Tech Cheerleading Team. Aside from this, she is passionate about exploring science through cooking and traveling.

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.